On coolant...especially HOATs & OATs types...
They are organic coating that needs an acid etch to get to bare metal
That means several hundred miles before the organic plates
That means metalized (oxides) liquid with more acid in the mix
So if your system was opened a long time and/or has an air leak...O2 is NOT
your friend and will react to form globules of acidic lumps to potentially clog
during the coolant cool down phase...inside the radiator cores of the main
and heater
Just a thought to make sure your coolant flow is good...along with checking
for debris reducing the cooling air flow
